This project investigates diagnosis and treatment of severe combined immunodeficiency, a rare inherited disorder also known as Bubble Boy disease. Affected infants have severe infections that are fatal unless the immune system can be restored. Bone marrow transplant (BMT) is life-saving if the disease is detected in time. SCID is most often caused by defects in the X-linked IL2RG gene that encodes the common gamma chain of receptors for cytokines. When this gene is defective, lymphocytes do not develop normally. To know how mutations in the IL2RG gene cause X-linked SCID (XSCID), we collect samples of blood or tissue, perform DNA analysis, assess expression of common gamma chain protein, and analyze its function. Mutations occur in all exons of the gene. Unusual mutations give clues to how the gene normally interacts with other proteins on the surface of the cell and in the cytoplasm of the cell. Certain mutations permit residual function and have distinct clinical characteristics. We look for other gene defects in patients with SCID but with no mutation in the IL2RG. There are 12 known SCID genes today. We perform carrier testing, genetic counseling and prenatal diagnosis, making affected infants eligible for improved early treatments. Despite improved survival with BMT, many XSCID patients are not completely cured, raising the question whether retroviral gene transfer ex vivo to autologous blood-forming cells could improve outcome. We have a complete XSCID gene therapy program, including vector development, animal models, retroviral transduction optimization, clinical evaluation of patients who have failed standard BMT treatment, and an active gene therapy protocol in which 3 patients have been treated. In a French gene therapy trial for XSCID, 8 infants had immune reconstitution, but 3 developed leukemia due to gene therapy vector insertion near an oncogene. Our trial is to treat only older patients who have failed standard bone marrow transplant treatment. SCID is just one of many diseases that could be approached by gene therapy to blood-forming stem cells. Our studies with gene transfer for treating SCID are a pilot application chosen because of the special biology of the IL2RG gene and its product, the common gamma chain. Cells with a common gamma chain have a selective advantage over cells without this receptor for growth and differentiation signals. If gene therapy can be made successful in SCID, the methods will be applicable to further human diseases of immunity such as SCID due to non-X-linked genes and HIV/AIDS. The true incidence of SCID is unknown because affected infants may die of infections without being diagnosed. The large number of samples we have received for mutation detection allows us to estimate the minimal incidence of SCID in the U. S. is at least 1/100,000 cases per year. Early identification of SCID by population-based newborn screening would enable more infants to be saved and would determine the actual disease incidence. We have developed a newborn screening test for SCID of any genotype by quantitation of circles of DNA made when the T cell receptor genes are cut and rejoined in developing T cells in the thymus). Our test has been validated on blood spots from the Maryland State newborn screening program. In addition, we have discovered a mouse with a new gene defect resulting in SCID. We will test humans with SCID, but who don't have defects in any known gene, to see if this gene also causes SCID in humans. Because this gene is a transcription factor that is important for T cell development it may have implications for future approaches to boosting T cell development in patients with HIV/AIDS.